A favorite trick of child psychologists is to put a baby on a table with a glass section in the middle. Will the baby scuttle across the transparent area, or does he instinctively know that it’s not a good idea to crawl over cliff edges? Curiously, despite repeated experiments, psychologists have not been able to agree on an answer. Babies are so active that it’s hard to tell whether those that do end up on the glass are innately foolhardy or have accidentally learned that it is safe. Still, whatever the truth about the tables,...